How to add Grand total to Matrix?

Hi,

I am not sure how your semantic model looks like, but I tried to create a sample pbix file like below.

One of ways to achieve this is to configure the format-setting of matrix visualization under "Row subtotal" section, like below image.

Please check the below Link (Microsoft Learn), picture, and the attahced pbix file.

Matrix visual format settings - Power BI | Microsoft Learn

Count measure: = 
IF ( NOT ISINSCOPE( result[category] ), BLANK (), SUM ( result[count] ) )

Stage measure: = 
SWITCH (
    TRUE (),
    ISINSCOPE ( result[Result] ), BLANK (),
    ISINSCOPE ( result[category] ), FORMAT ( SUM ( result[stage_count] ), "#,#0" ),
    FORMAT ( SUM ( result[count] ) / SUM ( result[stage_count] ), "#0.00%" )
)